1. Field of the Invention
The present invention relates to an image display and a method of driving the same, and more particularly, to a field sequential image display apparatus that reduces flicker and a method of driving the same.
2. Description of the Related Art
Examples of image displays that have an additional light source are flat panel displays such as liquid crystal displays (LCDs) and projection type displays such as liquid crystal on silicon (LCoS) devices and digital micro-mirror devices (DMD). These image displays can be widely used as monitors of computers and televisions.
LCDs, for example, display images by adjusting light transmittance of pixels after supplying voltages to each of the pixels on a liquid crystal panel according to input image signals. LCDs can be classified as red (R), green (G), and blue (B) color filter type LCDs, and field sequential driving type LCDs according to types of color images displayed.
In color filter LCDs, a unit pixel is divided into R, G, and B sub-pixels, and R, G, and B color filters are disposed on the R, G, and B sub-pixels respectively. Thus, light is transmitted to the R, G, and B color filters from one backlight unit to display color images. In color filter LCDs, the operation of the backlight unit is not linked with frame rate, and the backlight may be driven at a frequency so high that humans cannot detect. For example, in a conventional color filter LCD, the backlight can be driven at 150 Hz even when the frame rate is 60 Hz.
Meanwhile, in a field sequential LCD, R, G, and B lights transferred from R, G, and B backlights are displayed time-divisionally on the liquid crystal panel, thereby displaying color images using an after image effect. In order to sequentially display the image in a time-division manner, a field sequential LCD divides one frame of an image into R, G, and B fields and displays the R, G, and B fields on a screen sequentially. In a field sequential LCD, a resolution three times higher than that of a color filter LCD in a panel of the same size can be obtained, and moreover, the field sequential LCD has many advantages such as a large color gamut, absence of motion blur, low power consumption, and low fabrication costs due to the absence of the color filter processes.
In a field sequential LCD which typically has a frame rate of 60 Hz, when a frame is divided into three fields (R, G, and B fields), time allocated to a frame is 16.7 ms ( 1/60 s), and time allocated to a field is 5.56 ms ( 1/180 s). Changes of fields with the time interval of 5.56 ms, cannot be detected by human beings, and thus, the user recognizes three fields as a combined image in 16.7 ms and a color image by combining the R, G, and B colors is obtained.
However, since a conventional field sequential LCD supplies R, G, and B lights corresponding to the R, G, and B fields in the time-division manner, each of the R, G, and B backlights operates once in a frame. That is, the R, G, and B backlights emit light at the same frequency as the frame rate. For example, the R, G, and B backlights in a field sequential LCD having a frame rate of 60 Hz are driven at the frequency of 60 Hz.
FIG. 1A is a graph showing peaks of light intensity according to frequency in a color filter LCD including a backlight driven at a frequency of 150 Hz, and FIG. 1B is a graph showing peaks of light intensity according to frequency in a field sequential LCD including backlights driven at a frequency of 60 Hz. In FIGS. 1A and 1B, horizontal axes denote frequency coordinates, vertical axes denote light intensities, a solid line parallel to the x axes denotes −40 dB, and dotted lines parallel to the y axes denote 70 Hz or 100 Hz.
Referring to FIG. 1A, light intensity peaks over −40 dB are generated periodically at every 150 Hz which is the driving frequency of the backlight. Referring to FIG. 1B, light intensity peaks over −40 dB are generated periodically at every 60 Hz which is the frame rate. The periods in which the light intensity peaks are generated closely relates to the generation of flicker or color breakup by the backlight. If the light flickers at a high frequency, human beings cannot detect the flicker of the lights due to the afterimage effect. In general, in a case where the light intensity peaks are generated at every 70 Hz or higher, flicker cannot be detected, however, the light intensity peaks generated at a frequency of less than every 70 Hz, flicker can be detected by human beings. Referring to FIGS. 1A and 1B, there is no repeated light intensity peak within a range of ±70 Hz frequency or a range of ±100 Hz frequency in the color filter LCD, however, there are repeated light intensity peaks within a range of ±70 Hz frequency or a range of ±100 Hz frequency in the field sequential LCD. In general, broadcast images have a frame rate of 60 Hz, and thus, the flicker phenomenon is present in a field sequential LCD due to the flickering of the backlight.
Because the driving frequencies of the R, G, and B light sources are low in a conventional field sequential LCD, flicker is generated. Therefore, in a conventional field sequential image display using single color light sources such as R, G, and B light sources, the driving frequency of the light sources is the same as the frame rate, and thus, flicker is commonly generated due to the single color light sources. Meanwhile, a conversion of the frame rate in order to increase the driving frequency of the single color light sources requires additional circuitry, and thus, fabrication costs increase.